In a steering apparatus such as illustrated in FIG. 14 for applying a steering angle to the steered wheels of an automobile, the motion of a steering shaft 2 that rotates as a steering wheel 1 is operated is transmitted to an input shaft 6 of a steering gear 5 by way of a pair of universal joints 3 and an intermediate shaft 4. The steering gear 5 has a pinion that is rotated and driven by the input shaft 6, and a rack that engages with the pinion. As the pinion rotates together with the input shaft 6, the rack moves in the axial direction, which pushes and pulls a pair of tie rods 7 that are linked to both ends of the rack, and applies a desired steering angle to the steered wheels.
A gear housing 9 is connected to the bottom end of a steering column 8 inside which the steering shaft 2 is inserted, and this gear housing 9 supports an electric motor 10. The electric motor 10 applies an auxiliary force in the direction of rotation to the steering shaft 2.
On the other hand, in addition to the column-assist type of electric-powered power-steering apparatus illustrated in FIG. 14, there are also electric-powered power-steering apparatuses called pinion-assist type, dual-pinion type, and rack-assist type that are being used. FIG. 15 illustrates a steering apparatus in which a dual-pinion type electric-powered power-steering apparatus is assembled. In this dual-pinion type steering apparatus, a second input shaft 12 is located in a portion of part in the axial direction of a rack 11 that is separated from a pinion (first pinion) that is provided around the outer-circumferential surface of an input shaft (first input shaft) 6. The second pinion that is provided around the outer-circumferential surface of one end of the second input shaft 12 engages with the rack 11. An electric motor 10a is supported on the side of a housing 13 inside which the second input shaft 12 is housed. The electric motor 10a applies a force in the direction of rotation of the second input shaft 12 by way of a reduction gear 14. Therefore, the rack 11 moves in the axial direction according to the sum of a force that is based on an auxiliary force from the electric motor 10a, and a force by way of the input shaft 6 that is applied to a steering wheel 1 by an operator.
Rack teeth are provided in the axial direction along one side surface in the radial direction of this kind of steering gear rack except on both ends where the tie rods are connected. When constructing this rack, the rack teeth are formed by a cutting process, which together with increasing the manufacturing cost, makes it difficult to maintain the strength and rigidity of the rack teeth. On the other hand, by forming the rack teeth by plastic deformation, it is possible to reduce the manufacturing cost by shortening the time required for processing the teeth, and, the metal structure of the rack teeth becomes dense, so it becomes easy to maintain the strength and rigidity of the rack teeth. Methods for manufacturing a rack having teeth that are formed by plastic deformation are known, such as disclosed in JP H10-58081 (A), JP 2001-79639 (A), Japanese Patent No. 3,442,298, JP 2006-103644 (A) and JP 2008-138864 (A).
FIG. 16 to FIG. 21 illustrate an example of a conventional rack and manufacturing method thereof as disclosed in JP 2006-103644 (A). A rack 11a has a rod section 15 having a circular cross-sectional shape and that is made using a metal material such as carbon steel, stainless steel and the like, and rack teeth 16 that are formed by plastic working on one side surface in the radial direction of part in the axial direction of the rod section 15. In the example in the figures, the rod section 15 is integrally formed over the entire length using a metal material. Moreover, the radius of curvature R17 of the cross-sectional shape of the rear surface portion 17 on part in the axial direction of the rod section 15 that is separated in the circumferential direction from the portion where the rack teeth 16 are formed is greater than the radius of curvature r18 of the outer-circumferential surface of a circular rod section 18, which is the remaining part in the axial direction of the rod section 15 (R17>r18) (see FIG. 19). With this kind of construction, it is possible to make the rack 11a more lightweight by keeping the outer diameter of the portion other than where the rack teeth 16 are formed from becoming larger than necessary, while sufficiently maintaining the width dimension, strength, and rigidity of the rack teeth 16. The tooth depth of the rack teeth 16 (half the difference between the diameter of the tooth tip circle and the diameter of tooth base circle) is normally about 10% to 20% the diameter of the rod section 15.
In order to manufacture the rack 11a, first, as illustrated in FIG. 20A, a circular rod shaped raw material 19 is mounted into a concave groove section 21 having an arc-shaped cross section that is provided on the top surface of a receiving mold 20. Next, as illustrated in FIG. 20B, an upsetting process is performed by strongly pressing the raw material 19 toward the concave groove section 21 by the tip-end surface (bottom-end surface) of a pressure punch 22 that extends along the concave groove section 21. In this upsetting process, the portion of the raw material 19 where the rack teeth 16 are to be formed is squashed in the up-down direction, and the width dimension in the horizontal direction is widened to obtain and intermediate material 23. The intermediate material 23 has: a partial cylindrical-surface section 24 that will become the rear-surface portion 17, a flat-surface section 25 that is on the opposite side in the radial direction of the cross section from the partial cylindrical-surface section 24, and a pair of curved-surface sections 26 having a comparatively small radius of curvature that continuously connects the partial cylindrical-surface section 24 and flat-surface section 25.
Next, the intermediate material 23 is removed from the concave groove section 21, and as illustrated in FIG. 20C, is inserted into and placed at the bottom of a bottom section 29 of a support hole 28 that is provided in a die 27. The support hole 28 has a U-shaped cross section, and the radius of curvature of the bottom section 29 is nearly the same as the radius of curvature of the inner surface of the concave groove section 21 of the receiving mold 16. A pair of inside surfaces 30 is located on both sides in the width direction of the support hole 28, and these surfaces are flat surfaces that are parallel to each other. Furthermore, a pair of inclined guide surfaces 31 inclined in a direction so that the space between them increases going upward is provided at the opening on the top end of the support hole 28.
As illustrated in FIG. 20C and FIG. 20D, a teeth-forming punch 32 is inserted into the support hole 28 and this teeth-forming punch 32 strongly presses the intermediate member 23 inside the support hole 28. The processing surface (bottom surface) of the teeth-forming punch 32 has a shape that corresponds to the rack teeth 16 to be formed. Moreover, the outer-circumferential surface of the intermediate material 23, except for the flat surface section 25 where the rack teeth 16 are to be formed, is constrained by the inner surface of the support hole 28. Therefore, by the teeth-forming punch 32 strongly pressing the intermediate material inside the support hole 28, the flat surface section 25 of the intermediate material 23 is plastically deformed following the wave-shaped uneven surface on the lowered surface of the teeth-forming punch 32, and a raw rack 33 such as illustrated in FIG. 20D and FIG. 21A is obtained. However, the precision of the shape and dimensions of the raw rack 33, when compared with a completed rack 11a (see FIG. 16 to FIG. 19), is not sufficient, and the edge of the end with the rack teeth 16 remains sharp. Moreover, when processing the rack teeth 16, the excess material that is pushed out from the portion that will become the base of the teeth is strongly pressed against the inside surfaces 30 of the support hole 28, so a pair of flat flank surface sections 34 that are parallel to each other are formed on the left and right side surface of the raw rack 33.
After the teeth-forming punch 32 has been raised, the raw rack 33 is removed from the support hole 28, and as illustrated in FIG. 20E, is then placed on an uneven sizing surface 36 that is formed on the top surface of a sizing die 35. When doing this, the raw rack 33 is turned up side down. The uneven sizing surface 36 has a shape that corresponds to the rack teeth 16 to be obtained, including the shape of the chamfer sections on the end edges of the teeth. A stamp 37, as illustrated in FIG. 20E and FIG. 20F, strongly presses the portion of the raw rack 33 where the rack teeth 16 are formed toward the uneven sizing surface 36.
A concave pressing groove 38 having a radius of curvature that matches the radius of curvature R17 of the rear surface portion 17 of the completed rack 11a is formed on the bottom surface of the stamp 37, and with the portion that will become the rear surface portion 17 fitted inside the concave pressing groove 38, the raw rack 33 is strongly pressed toward the uneven sizing surface 36. With the sizing die 35 and stamp 37 sufficiently close together as illustrated in FIG. 20F, the rack teeth 16 are formed into the completed state illustrated in FIG. 21B, or in other words, the shape and dimensions thereof become proper, while a chamfer is provided on the end edges of each of the teeth, and the shape and dimensions of the rear surface portion 17 become proper at the same time. The excess material that was pressed out by this sizing process is collected in the pair of flat surface sections 34. Therefore, the pair of flat flank surface sections 34 hardly remains in the completed rack 11a. When doing this, the extra material does not apply extremely strong pressure against the uneven sizing surface 36 or concave pressing groove 38, so the processing load during sizing is kept low, and it is easy to maintain the durability of the sizing die 35 and stamp 37.
However, in the case of the conventional manufacturing method for a rack, there is a possibility that problems such as the following could occur. In other words, as illustrated in FIG. 20C and FIG. 20D, when the teeth-forming punch 32 strongly presses the intermediate material 23 inside the support hole 28, part of the metal material of the intermediate material 23 moves toward the outside in the axial direction from the end section in the axial direction of the portion where the rack teeth 16 are to be formed (portion separated in the axial direction from the power pressed by the teeth-forming punch 32) as the teeth-forming punch 32 presses the material. As a result, as illustrated in FIG. 22, the depth of the rack teeth 16 becomes smaller on the end section in the axial direction of the rack teeth 16 going toward the outside in the axial direction, and when the completed rack is assembled in the steering gear 5, there is a possibility that on the end in the axial direction of the rack teeth, the engaged state between the rack teeth 16 and the pinion that is formed around the outer-circumferential surface of the input shaft 6 will not be able to be maintained.
On the other hand, as illustrated in FIG. 23, it is possible to provide dummy teeth (excess section) 39, 39a in the portion of part in the axial direction of the rod section 15a that is adjacent to the portion where the rack teeth 16 are to be formed and having a tooth depth that is less than that of the rack teeth 16 so that there is no engagement with the pinion that is formed around the outer-circumferential surface of the input shaft 6 even when the steering gear 5 is in use. As a result, the tooth depth is prevented from becoming insufficient on the end section in the axial direction of the rack teeth 16, and it is possible properly maintain an engaged state between the rack teeth 16 and the pinion on the input shaft 6. However, with this construction, by providing dummy teeth 39, 39a, there are problems in that the length in the axial direction of the portion where plastic deformation is performed increases, and it is necessary to also increase the length in the axial direction of the processing tools (punch and die). Moreover, there is also a problem in that it becomes impossible to sufficiently maintain the thickness between a screw hole 57 for screwing in a male screw section of a ball joint for connecting to the tie rod and the dummy teeth 39a that are provided on the screw hole 57 side.